1. Field of the Invention
The present invention is related to an automatic depth compensating system and the incorporation of the automatic depth compensating system as an improvement in a rotary drill bit and more particularly, to an automatic depth compensating system which has at least one pressure compensating device for changing the pressure in the interior of a volume in response to pressure changes in the environment surrounding the volume and a pressure surge compensating device which compensates for positive and negative pressure surges when the pressure within the volume changes is relative to the environment. Furthermore, the present invention is more particularly directed to a rotary drill bit which includes the improvement of this type of automatic depth compensating system.
2. Description of the Prior Art
In all rotary drills used in the drilling of wells in the ground as well as many other types of devices used in wells which are either drilled or being drilled, there is a need to protect the inner working parts of the drill and devices from pressure and from the environment in which the drill device is working as the environment is often very aggressive or damaging. The need for protection is most critical at points where there is relative movement between different parts of the device. Typically, problems of this type occur in the movement of drilling cones of tricone drill bits and in the movement of a single cutting head relative to the drill bit body as in the Barnetche drill bit such as that shown in U.S. Pat. No. 4,154,312. The problem with regard to protection also exists in devices other than drill bits as, for example, in downhole motors, downhole pumps, etc.
In devices such as those noted above, since there is movement between parts, there is also a need for bearings of some type. The bearings in turn require a clean self-contained lubrication system and the fact that the relative movement occurs between different parts of these devices creates a need for a seal between the separately moving parts which will isolate the internal lubrication system from the downhole environment. It is these seals which are usually the most vulnerable elements in the devices. In actual drilling, these are a number of adverse factors which exist simultaneously. Furthermore, many of these conditions exist in the downhole environment even after drilling is completed and thus have an impact on other types of downhole devices as well as drills.
One factor contributing to the performance requirement for seals on downhole devices is pressure. Due to the need for circulation of drilling fluids or mud which are commonly employed in rotary drilling of deep wells, the pressure on devices at the bottom of the well are extreme. If the internal pressure of the self-contained lubrication system of the downhole device were to be at atmospheric pressure the seals on the device would be required to withstand tremendous differential pressures since the downhole pressure of the environment is very high. Further, the environment at the bottom of the hole is extremely aggressive or damaging due to the presence of drilling fluid, mud, additives, drill cuttings, etc. The environment, thus, would produce a very erosive action on the seals. Failure of the seals under the large differential pressure load will result in leakage of the aggressive downhole element through the seal and into the device itself, producing rapid deterioration and failure of the internal components, such as the bearings.
In order to provide compensation for the extreme downhole pressures and to reduce the damaging effect of the aggressive environment on the seals, pressure compensators have been used. An example of a pressure compensator can be found in U.S. Pat. No. 4,154,312 which is directed to the Barnetche bit, this patent being incorporated herein by reference. In the pressure compensator disclosed in this patent, a diaphragm is positioned between the interior and exterior of the downhole device in order to equalize the pressure between the volume inside the drill bit and the environment. In the prior art a simple piston in a cylinder has been used instead of the bellows. This type of pressure compensator reduces the pressure differential to 0. However, pressure compensators such as that disclosed in the Barnetche bit patent, have the disadvantage that at best, they produce equal pressure between the inside of the drill bit and the environment. Furthermore, generally due to hysteresis and viscosity of internal lubricants, the internal pressure is less than the external pressure. This means that at the seal in the case of the differential pressure being 0, the seal tends to dry out since no lubricant is added to the seal faces. In the case of negative internal pressure even though it is small, there will be a small amount of leakage in which the aggressive external environment will enter the seal faces thereby acclerating seal wear. Furthermore, the harmful elements of this aggressive environment will also have a damaging effect on the internal parts of the drill bit such as the bearings.
A drill bit lubrication system using a differential piston is shown in U.S. Pat. No. 3,244,459. The differential piston includes a cylinder of two different diameters with a piston having different diameters in each of the portions of the cylinder. This type of differential piston will provide an internal pressure greater than the external pressure and in fact, the internal pressure will always be a predetermined percentage greater than the external pressure. The differential piston device of Ortloff, however, has two significant disadvantages. First, since the internal pressure is a predetermined percentage greater than the external pressure, at extreme operating pressures which are often found in well drilling such as the type with which it is used, although the percentage may be small, the absolute pressure differential between the internal and external pressure is very large. This large pressure differential cannot be maintained by the seals and the lubricant within the interior of the drill bit is forced through the seal to the exterior and eventually there is not sufficient lubricant to provide the required lubrication for the drill bit. In other words, the drilling depth is limited when using a differential piston device such as that shown in Ortloff. Another significant disadvantage in the device of Ortloff, is that because of the large number of seals required between the piston and cylinder, there is large static friction which must be overcome before there is movement of the piston. The piston does not react to pressure surges unless they are very high and, thus, there can be a leaking of the seals during the periods of pressure surge.